An electrochemical, catheter-mounted gas sensor is proposed for concurrent and independent measurements of pCO2 and pO2 in blood. This sensor will also be investigated for the simultaneous measurement of pN2O. The CO2 and O2 measurements will be of interest for ventilation management, acid-base balance and basic research, while a sensor with capability for N2O sensing could be very helpful in the operating room to determine the anesthetic depth. The principle of the sensor is based on the determination of the amount of "Reduced-CO2" adsorbed on the Pt working electrode ring. pO2 is obtained by measuring the O2 reduction limiting current in a well defined potential region. The pCO2 and pO2 measurements can be made at a high sampling rate without mutual interference.